Glass-ceramic plates, method of manufacturing them and hobs equipped with these plates

ABSTRACT

A glass-ceramic plate, a method of manufacturing a glass-ceramic plate, and hubs equipped with the glass-ceramic plate. A flat or substantially flat glass-ceramic plate includes a surface provided with pegs, the plate configured to equip a hob. At least one smooth region, free of pegs, is reserved in a location designed to come, in a mounted position, face to face with one or more elements of the hob, the blurred view of which due to the pegs must be improved.

The present invention relates to hot plates made from a glass-ceramic orfrom another similar material able to withstand high temperature andhaving a zero or almost zero expansion coefficient.

These plates, which are flat or essentially flat, are designed to coverheat elements or sources so as to form hobs. The heat sources are ofvarious types: electrical heating elements, such as elements for heatingby electrical resistance or radiant elements and halogen lamps;inductors or induction sources combining to bring about inductionheating; and gas burners, it being possible for the heat sources of oneand the same hob to be of one and the same type or of different types,such as radiant elements and gas burners (hobs referred to as “mixed”hobs).

Hobs also comprise control and adjustment means making it possible tovary the power of the sources (of which there are usually four) andpossibly various indicators or lights.

These glass-ceramic hot plates or the like have seen immense success andare very broadly widespread especially since they benefit from anexternal appearance which is more modern than the conventionalelectrical plates and from ease of cleaning because the surface of aglass-ceramic plate is flat or essentially flat.

The term “essentially flat” is understood to mean that the glass-ceramicplate is flat but may also comprise, in some locations, deformationsconsisting either of concavities or hollow regions, or of bosses orreliefs or ridges.

Thus, mention may be made of the plates of the type described in Frenchpatent application FR-A-2 773 873 which comprises at least one concavityunder which a heat source is placed and in which food can be directlycooked or a cooking container can be placed, it being moreover possiblefor the upper peripheral edge of the concavity to be raised from theplane of the plate.

Mention may also be made of the plates according to EP-A-0 930 806 whichcomprise hollows and reliefs making it possible for a visuallyhandicapped user to find the location of a region and/or of a particularfunction of the plate by touch.

Also, mention may be made of the plates according to WO 01/38796 whichpossess deformed parts forming the top parts of the combustion chambersof gas burners, these deformed parts having slots for the output offlames and intake of air.

To produce the control and adjustment means making it possible to varythe power of the heating elements, traditional knobs fastened to axespassing through the holes made in the plate are now generally replacedby electronic touch-sensitive pads or capacitive pads, actuated bycontact with a finger. The means for controlling and adjusting a meansfor heating or for maintaining temperature for example consist of a pairof pads, one of which serves to increment the power delivered to thesource on each contact of the user's finger on this pad, while incontrast, the other serves to decrease stepwise the power of this samesource. A multiplicity of pads corresponding to different heating powersmay also be provided.

The glass-ceramic plate therefore no longer needs to be pierced as forcontrol using knobs. It is enough to define the pads on the upper partof the plate, since just finger contact creates a capacitance with theother branch of the capacitor which is under the plate, theglass-ceramic acting as a dielectric.

In general, manufacture of the glass-ceramic plates comprises thefollowing successive steps:

-   -   Melt-rolling: in a melting furnace, glass of a composition        chosen in order to form the glass-ceramic is melted, then the        molten glass is rolled into a standard ribbon or sheet by        passing the molten glass between two rollers, generally an upper        roller with a smooth surface for the purpose of obtaining a        smooth upper surface for the plate and a lower negatively-etched        roller, so as to obtain a lower plate surface provided with a        network of bosses or pegs, for example hemispherical or        semielliptical bosses or pegs. The height of these pegs is in        particular about 0.10-0.30 mm; the distance between the pegs is        about 1 to 5 mm; and the pegs are advantageously arranged in        staggered rows; the roller with pegs is advantageously the lower        roller, the pegs then facilitating driving;    -   Cutting the glass ribbon to the desired dimensions; as a general        rule, two plates will be cut from the same ribbon;    -   Possibly formation by screen printing, generally on the upper        surface of the plate and according to the design provided for        each plate, of limits of the heating regions, of visual        indicators corresponding to the location of the touch-sensitive        pads and to their allocation to the various sources, of        decoration (affixing the manufacturer's or vendor's mark) and of        any desired patterns;    -   Ceramization: the plates, generally after cutting and possibly        after screen printing are then baked according to the thermal        profile chosen in order to transform the glass into the        polycrystalline material called a “glass-ceramic” whose        expansion coefficient is zero or almost zero and which        withstands thermal shock which may go up to 700° C., while at        the same time baking the enamel of the screen-printed patterns        thus enabling them to be attach to the substrate.

When the glass-ceramic plate comprises shaped regions (raised regions,hollow regions, etc.), the manufacturing method comprises an operation,which can be carried out at any time during the conventional method, ofcollapsing, molding or pressing using a support or molding or pressingsurfaces having the desired geometry, provided that the material formingthe plate is at a sufficiently high temperature to allow the plasticdeformation thereof.

The present invention relates to all the plate types which have justbeen described, which could if necessary have borings, and/or possiblyadded elements such as frame, connector(s), etc.

It is therefore noticed that these glass-ceramic plates have a generallysmooth upper surface and a lower surface generally provided with pegs asdescribed previously.

The upper surface is preferably smooth for electrical plates in order toallow good contact with the cooking containers and to transfer heatunder the best conditions and in all cases for ease of cleaning and theesthetic appearance. This surface will be protected when the plates aretransported.

The lower surface is provided with pegs (or bosses or studs or raisedparts) so as generally to improve the mechanical strength of the plate.The pegs also ensure that there is no contact with the support plateduring ceramization. In the event of shock on the top of the plate, thelower surface of the plate will be put in extension; if there is adefect in the plate, the microfissure will not propagate due to thepresence of the pegs. Moreover, in the event of scratching, the latterwill be discontinuous and will therefore be less visible and in allcases will be invisible if looking at it from the top of the plate. Thepegs therefore provide useful passive protection.

At the same time, for a hob with halogen sources, the latter appear morecloudy and more blurred because of the presence of pegs, which has thebenefit that the users are not dazzled by these elements.

However, although this blurring effect is beneficial for halogenelements, it is much less so for display modules, for example displaymodules known as “7-segment display modules” which are associated withtouch-sensitive control pads, and which indicate the heating power of agiven heating source to the user. Indeed, even when the material formingthe plate is very weakly absorbing or diffusing with respect to a lightsource (for example, the transmission, through a plate of the Kerablack®type, of the red light from a “7-segment” display module is very good),the view of this source is deformed because of the pegs. The result ofthis is that, on the one hand, the view is not esthetically pleasing andthat, on the other hand, reading the figures (or else messages, forexample, with regard to the state of the cooking) may be so bad as to beillegible and could cause errors when cooking dishes, or even accidents.

To overcome this drawback, it is known and it is common current practiceto apply a resin (such as silicone) having the same optical index as theglass-ceramic to the lower part of the plate so as to fill the hollowsbetween the pegs in the regions designed to come face to face with thedisplay modules in the assembled position of the hob. Thus a smoothsurface is formed at this location, making it possible to improve theview of the display modules, without however completely removing thisblurred vision.

It is also necessary to stress that the application of the index resinrepresents an additional step, at a later stage, consisting ofhigh-accuracy work, with particular storage conditions (since thesilicone polymerizes in 24 hours in dust-free air). This operation istherefore expensive. Furthermore, the drawback of poor ageing of theresin should not be excluded.

The filing company has therefore sought to provide a satisfactorysolution to this problem, which has led it to modifying the surface ofthe roller with pegs in order to remove the pegs over at least oneregion chosen in order to keep, over the lower surface of theglass-ceramic plate, at least one peg-free region avantageously in alocation designed to come face to face with an element such as a displaymodule which it must be possible to see from above the plate withoutblurring.

Thus, the present invention makes it possible to overcome the drawbackof the blurred view of the display modules by manufacturing plates whichare partially free from pegs on the lower face, without howeverimpairing the mechanical strength of the whole plate.

The subject of the present invention is therefore firstly a flat orsubstantially flat glass-ceramic plate comprising a surface providedwith pegs, said plate being intended to equip a hob, characterized inthat at least one smooth region, free of pegs, has been reserved in alocation, this location being advantageously intended to come, in themounted position, face to face with one or more elements of said hob forwhich the blurred view of which due to the pegs must be improved.

Such elements are in particular power display modules for example thoseknown as “7-segment display modules”, or other indicators, especiallytext indicators designed to be read, for example giving informationconcerning a cooking program to be followed, or else various lights.

Preferably, the smooth region of the surface provided with pegs of theplate is parallel to the other surface of the plate. The sought-afterview then becomes clear since both faces of the plate are parallel inthe viewing region.

According to the present invention, the smooth region of the surfacewith pegs of the plate may be located in the plane of the projectingends of the pegs; or in the plane of the bottoms of the hollow regionsbetween the pegs; or in a plane intermediate between the plane of theprojecting ends of the pegs and the plane of the bottoms of the hollowregions between the pegs. In the second case, an improved view will beobtained, insofar as the plate is thinner, which is sought after for adisplay module.

The pegs generally have a height of 0.10 to 0.30 mm.

According to a particular embodiment of the glass-ceramic plateaccording to the invention, the surface with pegs of the lattercomprises a smooth region, without pegs, which region is formed by astrip parallel to one of the edges of the plate, near to said edge, saidstrip corresponding to the area of a series of display modules fordisplaying heating powers or other information for the user. The surfaceopposite the surface with pegs of the plate may also have a boss or agroove parallel to the strip without pegs in the neighboring area of thelatter opposite said edge.

The formation of the strip without pegs, notably by the rolling asmentioned in the present invention, can also induce on the surfaceopposite to the surface with pegs of the plate the presence of one ormore undulations or waves (especially of the order of 1 to a fewmicrometers of height compared to the plane of the plate), facing (orabove) the area without pegs and parallel to the strip without pegs, theplate according to the invention and obtained by the rolling asmentioned, is consequently to be generally distinguished from theexisting products not only by a least one area without pegs but also bythe presence, on the opposite surface, facing the area without pegs, ofthis or these undulations or waves (generally a double range ofundulations). Yet, it is also possible, by modifying the surface (forexample by machining) of the second roller (that of the face opposite tothe one with pegs, generally the upper roller) to reduce or remove theseundulations. It is also possible to form one or more additionalundulations, and/or one or more bosses or waves or raised parts orprofile variations not disturbing the vision (these variations canpossibly go from 0.1 to 3 mm of height), in neighbouring or facingarea(s) (or possibly not) compared to the area without pegs and on theopposite face; for example, it is possible to form a wave on the edge ofthe area facing the area without pegs on the opposite face, this wavediminishing the phenomena of undulations due to the rolling process.

According to a mode of realization of the glass-ceramic plate accordingto the invention, the smooth region or regions of its surface with pegsare protected during transport of the plate by means of a polymer, forexample a peelable transparent protective sheet made of polyethylene,which is removed when assembling the hob.

The present invention also relates to a hob, in particular of theradiant element, halogen element, induction or gas burner type or ofmixed type, said hob being equipped with a glass-ceramic plate asdefined above, the peg-free region or regions corresponding especiallyto the areas of the display modules for displaying the heating powers orother information useful to the user.

The subject of the present invention is also a process and an apparatusfor manufacturing, by melt-rolling, a glass-ceramic plate as definedabove, comprising respectively a rolling step and a rolling deviceconsisting of two rollers between which the molten glass to be rolled ispassed in order to obtain a ribbon whose length corresponds to one ormore plates, one of the rollers, generally the lower roller, comprisinga surface with pegs, characterized in that the roller with pegs has beenmodified in order to comprise at least one smooth region free of pegs,the smooth region or regions on the surface of said roller having beendimensioned and positioned in order to form, during rolling, at leastone peg-free region on the surface of the ribbon, dimensioned andpositioned generally in order to be, after the operation of cutting theribbon into plates having their final dimensions, face to face with oneor more elements of said plates the blurred view of which due to thepegs must be improved.

In particular, a process or apparatus according to the inventionintended to form a plate whose smooth region is a strip as was describedabove is characterized in that the peg-free region of the lower rollerconsists of a strip or a portion of cylindrical strip which is locatedclose to an edge of the roller, in order to obtain in this way the lowerpeg-free strip parallel to the edge of the plate.

To better illustrate the subject of the present invention, particularembodiments will now be described with reference to the appendeddrawings in which:

FIG. 1 is a schematic top view of a glass-ceramic hob of the invention;

FIG. 1 a is an enlarged view of a region for adjusting the heating powerof a heat source of the hob of FIG. 1;

FIGS. 1 b and 1 c are detailed views of the upper part of the 7 segmentdisplay module, as it appears respectively with and without the means ofthe present invention;

FIG. 1 d is a sectional view along 1d-1d of FIG. 1, FIGS. 1 e and 1 fshowing two variants;

FIG. 2 is a perspective view of a lower roller, modified according tothe present invention;

FIG. 2 a is, on a larger scale, a partial sectional view of the rollerof FIG. 2, showing the peg-free region of the roller, and thecorresponding area of the glass-ceramic plate; FIG. 2 b is a variant ofFIG. 2 a;

FIG. 3 is a view showing the side with pegs of the glass ribbon obtainedafter rolling with the lower roller of FIG. 2, the cutting lines of theplates being shown by mixed lines;

FIG. 4 is a view similar to FIG. 3 showing a ribbon obtained with aroller made according to a variant of the roller of FIG. 2.

In FIG. 1, a schematic view of a hob 1 according to the invention isshown, comprising a flat glass-ceramic plate 2 on which the regions 3 to6 of the four heat sources have been defined conventionally, by screenprinting and, on the lower edge, the regions 3 a to 6 a of thetouch-sensitive pads controlling the heating power of the respectivesources. The upper part of the plate 2 may comprise, along its loweredge, a ridge 7 or a groove 7′ as can be seen in FIGS. 1 e and 1 frespectively, which separate the major area comprising the regions ofthe sources and the edge strip which comprises the touch-sensitive pads.

The glass-ceramic of the plate 2 is colored (for example brown, white)and when the plate 2 is in service, each of the display modulesequipping the hob 1 under each of the touch-sensitive pads 3 a to 6 acan be seen by transparency, as shown in FIG. 1 a, together with thegeneral switch 8.

The display module shown in FIG. 1 a is a conventional 7-segment displaymodule, the figure formed showing the heating power, the user pressingseveral times to the right on the + in order to increase the power or onthe − in order to decrease the latter.

The figure appears clearly in FIG. 1 b which illustrates the inventionand blurred in FIG. 1 c which illustrates the prior art. This isbecause, as explained above, the lower surface of the plate 2 isprovided with a network of pegs 9 (see FIG. 1 d) over its entire surfacefor a conventional plate, while these pegs 9 have been removed in thestrip 10 facing the display modules in the plate 2 according to theinvention.

In the case illustrated, the plane of the strip 10 is that of thebottoms of the hollow regions between the pegs 9, which provides aperfect view of the display modules.

This strip 10 parallel to the upper surface of the plate 2 is obtainedby using a negatively-etched lower roller made of steel (with a hollowregion depth of for example d=0.2 mm, which will give pegs of 0.14 mmhigh), except over a strip 12 close to one of its edges which is smooth.Rolling with a set of rollers (smooth upper roller, not shown, and lowerroller 11) of a glass ribbon 13 in which two plates 2 are capable ofbeing cut, will form the sought-after smooth strip 10 (FIG. 3).

The glass ribbon of FIG. 4 will be formed with a roller comprising tworegions free of pegs, in order to form two plates which are head totail.

As can be deduced from FIGS. 2 a and 2 b, the depth of the smooth regionof a lower roller 11 may vary from 0 to d. For the variant of FIG. 2 a,the roller has been machined at a later stage thereby removing the pegsin the smooth region. For the variant of FIG. 2 b, a roller 11′ etchedwith pegs except in the smooth region 12′ has been provided for.

It is of course understood that the particular embodiments describedabove have been given by way of non-limiting indication and thatmodifications and variants may be provided without in any way departingfrom the scope of the present invention. In particular, the surface withpegs has been described as being only the lower surface, but where theupper surface would have, or would also have, irregularities of the sametype, the same solution of smooth region(s) corresponding to the areathrough which a clear view of the lights and display modules, etc. isdesired, is to be recommended, and therefore a suitable treatment of thecorresponding roller.

1. A process for manufacturing a glass-ceramic plate comprising:providing a rolling device including two rollers between which moltenglass to be rolled is passed; obtaining a ribbon whose lengthcorresponds to one or more plates by passing the molten glass betweenthe two rollers, one of the rollers including a surface with pegs,wherein the roller with pegs includes at least one smooth region free ofpegs, the at least one smooth region on the surface of the roller beingdimensioned and positioned to form, during rolling, at least onepeg-free region on a surface of the ribbon, the peg-free region on thesurface of the ribbon being dimensioned and positioned to be, after anoperation of cutting the ribbon into plates having final dimensions,face to face with one or more elements of a hob on which the plate isintended to be placed.
 2. The process according to claim 1, formanufacturing a glass-ceramic plate, wherein the peg-free region of theroller includes a strip or a portion of a cylindrical strip locatedproximate to an edge of the roller to obtain a peg-free strip parallelto an edge of the plate.
 3. An apparatus for manufacturing aglass-ceramic plate, comprising: a rolling device including first andsecond rollers between which molten glass to be rolled is passed toobtain a ribbon whose length corresponds to one or more plates, thefirst roller including a surface with pegs, wherein the first rollerincludes at least one smooth region free of pegs, the at least onesmooth region on the surface of the roller being dimensioned andpositioned to form, during rolling, at least one peg-free region on asurface of the ribbon, the peg-free region on the surface of the ribbonbeing dimensioned and positioned to be, after an operation of cuttingthe ribbon into plates having final dimensions, face to face with one ormore elements of a hob on which the plate is to be placed.
 4. Theapparatus as claimed in claim 3, wherein the peg-free region of thefirst roller includes a strip or a portion of cylindrical strip locatedproximate to an edge of the roller, the strip or portion of acylindrical strip being configured to form a peg-free strip on the plateparallel to an edge of the plate.
 5. A process for manufacturing aglass-ceramic plate, comprising: rolling molten glass between first andsecond rollers to obtain a ribbon whose length corresponds to one ormore plates configured to fit a hob, the first roller comprising asurface with pegs and at least one smooth region free of pegs; formingpegs on a surface of the ribbon with the first roller; and forming atleast one peg-free region on a surface of the ribbon with the firstroller, the at least one peg-free surface dimensioned and positioned tooverlap, after an operation of cutting the ribbon into plates havingfinal dimensions, one or more one or more elements of a hob on which theplate is to be placed.
 6. The process according to claim 5, furthercomprising forming a peg-free strip parallel to an edge of the plate viaa peg-free region of the first roller.
 7. The process according to claim6, further comprising providing the peg-free region of the first rolleras a portion of a cylindrical strip located proximate to an edge of thefirst roller.